Semiconductor integrated circuits often incorporate hundreds of thousands of semiconductor elements on a single chip. These elements are interconnected to perform a desired function.
Changes in process, voltage and/or temperature (PVT) of an integrated circuit chip can cause changes in propagation delays through the elements of the circuit. These changes can therefore have a direct negative impact on the timing margin of a particular design and can affect its functional operation.
For example, integrated circuits typically have one or more interfaces for communicating with other devices. Some integrated circuits incorporate high speed interfaces, which usually have a clock recovery unit or a delay line of some kind that is adapted to adjust for changes in delays, such as on-chip delays or on-board delays, due to changes in process and factors.
Certain interfaces, which are mainly chip-to-chip interfaces on the same board, are defined to be source-synchronous interfaces. These types of interfaces typically do not have a clock/data recovery unit, but rather latch the received data using the synchronous clock that is provided with the data from the transmitter side of the interface. A fixed delay is used to shift the received clock signal so that the shifted clock transitions in the middle of the data eye and can therefore be used to latch the data. However, the fixed delay can introduce a certain level of uncertainty since under worst-case process, voltage and/or temperature (PVT) conditions, the fixed delay can be different (e.g., longer) than under best case conditions. Therefore, it can become difficult to design high speed interfaces with a large timing margin.
Preferably, delays should remain constant from one integrated circuit to the next and over changes in the fabrication process and in operating conditions such as operating voltage and temperature. A constant delay can provide a larger timing margin.
Improved methods and apparatus are desired for compensating changes in delays caused by changes in process, voltage and/or temperature of an integrated circuit chip.